1. Field of the Invention
The invention concerns a pneumatic suspension leveling system for vehicles, in particular for motor vehicles, with a selectively activated air compressor having a pumping capacity more than sufficient for normal demands of the suspension leveling system. Such a pneumatic suspension leveling system is known to the art, for example, from DE 196 21 946 C1.
2. Description of Related Art
Pneumatic suspension leveling systems for motorized vehicles such as, e.g., trucks and their trailers as well as buses, are fundamentally known to the art and are utilized on a mass-production basis. They serve vehicle suspension and are additionally used for vehicle level adjustment. With some designs of pneumatic suspension leveling systems of this kind, a pressure accumulator is provided between the compressor outlet and the suspension leveling devices and which is constantly maintained at a boost pressure which lies distinctly above the operating pressures of the pneumatic suspension leveling elements and/or air suspension bellows. With such a pneumatic suspension leveling system the compressor operates as a rule only when the pressure accumulator must be recharged, in which case compressor output can be below peak capacity during peak pneumatic suspension leveling system demand because an adequate pressure supply is always assured by the pressure accumulator.
When using electrically driven compressors, the compressor heats up markedly during with continuous operation over longer periods of time, which leads to compressor overload and damage and, consequently, can lead to pneumatic suspension leveling system failure. In order to prevent compressor overload of this kind, a predetermined maximum on-time is prescribed by an electronic control device of the pneumatic suspension leveling system.
During compressor operation, a distinction is made between relative on-time duration and an absolute on-time duration. Relative on-time duration indicates the proportion of the on-time of an operating period, whereby this operating period is composed of the off-time and an off-time which must be maintained between two successive on-times. For example, a relative on-time duration ED.sub.Tel of 20% means that an off-time of 40 seconds must first follow an on-time of, e.g., 10 seconds before the compressor may be switched on again. In the aforementioned example, the operating period turns out to be 50 seconds, the sum of the on-time (10 seconds) and off-time (40 seconds).
In contrast thereto, the absolute on-time duration determines the maximum on-time which the compressor may be allowed to operate without interruption (off-time). The speed with which the compressor heats up during its operation is a function of different limiting conditions such as, for example, mounting position of the compressor on the vehicle, air temperature, compressor air flow charging speed. The operating conditions resulting from these limiting conditions for the compressor can delay or accelerate compressor heating.
The relative on-time duration and the absolute on-time duration preset in the control device of pneumatic suspension leveling systems known to the art can lead to unfavorable operating conditions for the compressor in order to prevent exceedingly short off-times and, consequently, can result in overheating of the compressor during unfavorable operating conditions. However, operation of this design with relatively long off-times does not provide sufficient pumping capacity for operating the pneumatic suspension leveling system.